1. Field of the Invention
The present invention relates generally to glass enamel compositions which may be fired upon a glass substrate, and, more particularly, to a glass enamel composition containing aluminum and/or lithium oxide to prevent sticking of the enamel to molding surfaces for the glass substrate and as migration preventor to prevent the migration of a finely divided material such silver powder to the glass substrate.
2. Description of the Prior Art
In recent years, the automobile industry has manufactured automobiles with back windows which include electrical heating elements to remove frost formed on the window surface. The back windows, sometimes referred to in the industry as back lights, are printed by a silkscreen printing process with a grid of a metallic material which is then fired on the glass window to form the electrical heating element. In most instances, the grid arrangement forming the heating element is comprised of a bus bar extending along each side of the window, and a series of fine lines extending horizontally across the window, with the fine lines being connected to the bus bars. The grid material from which the heating element is formed typically of a mixture containing a silver powder and a small amount of soft-lead glass dispersed in a printing vehicle, such as oil, suitable for silkscreen printing. The grid material is applied to the glass substrate in a silk-screen printing process.
Also frequently applied to the back window is a dark grey or black border extending about the periphery, or outer edge, of the back window. The border is printed both for aesthetic reasons, and, also, to protect an adhesive utilized to seal the back window to the car body from degradation due to exposure to ultraviolet light.
In some instances, the bus bars of the heating element are printed on the back window next to the paint film edge of the border, but the visibility of the bus bars is aesthetically undesirable. In other instances, the bus bars are printed over the paint film of the color border, but, after firing, the bus bars are still visible, and appear from the outside of the automobile as a dark amber color. Not only is this design aesthetically undesirable, but printing of the bus bars over the color borders reduces the electrical conductivity of the bus bars, and also reduces the strength of the solder joints which must be applied to the bus bars to couple external electrical wires thereto.
Another problem associated with printing the bus bars upon the color border is staining of the glass. When the grid material containing silver is fired, an ion exchange occurs resulting in silver ions migrating through the ceramic paint and producing a dark amber stain in the glass.
One suggestion to prevent the detection of the bus bar from outside of the automobile is to alter the composition of the paint utilized to form the border. In particular, it has been proposed to add powdered zinc, tin, cadmium, or manganese to the ceramic color paint. The metal powder must be very fine, and have an average particle size of about one micron.
Because aluminum oxidizes at low temperatures, and oxidizes even more readily at higher temperatures, in the past, it had been thought that the use of aluminum as the metal powder to be added to the ceramic paint was not feasible, as the ceramic paint utilized to form the ceramic color border is usually fired at a temperature in excess of 1,100 degrees Fahrenheit.
Disclosed in U.S. Pat. Nos. 4,684,388 and 4,684,389 are means to form a glass sheet having an oil base ceramic paint fired thereupon wherein the ceramic paint contains a fine zinc metal powder. U.S. Pat. No. 4,684,388 further discloses the inclusion of a fine stannous oxide powder in an ultraviolet curable ceramic paint which, when applied to the glass sheet, is subjected to ultraviolet radiation and heated to a temperature to soften the glass sheet to allow bending thereof. The ceramic paint on the glass engages with a fiberglass covering on a forming die. The zinc and the stannous oxide powders serve to prevent the sticking of the ceramic paint to the fiberglass forming die. U.S. Pat. No. 4,684,389 discloses an oil base ceramic paint to which the fine zinc powder is added to the paint applied to the glass sheet. The painted glass sheet is then heated to a forming temperature and engaged with a fiberglass covering of a die to form a glass sheet of a desired shape. Again, the metal powder functions to prevent the sticking of the ceramic paint to the fiberglass of the forming die during the forming process.
It is therefore an object of the present invention to provide a method of applying glass enamel having a particular composition which is economical and avoids the problems with electrical conductivity, aesthetic appearances, and ion migration associated with prior art glass enamels utilized in silk-screen printing operations.